These steps will touch up a non-root install...
libexec_bin=$(spack location -i singularity)/libexec/singularity/bin
suid_files=(action-suid start-suid mount-suid)
for f in "${suid_files[@]}"
do
sudo chown root:root "${libexec_bin}/${f}"
sudo chmod 4755 "${libexec_bin}/${f}"
done
sudo chown root:root $(spack location -i singularity)/etc/singularity/singularity.conf